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The invention relates to electrostatic accelerators in general and to the use of electrostaic accelerators to perform accelerator mass spectrometry in particular.
Since the late 1970""s techniques have been developed for using tandem electrostatic accelerators to develop extremely sensitive mass spectrometers able to distinguish the presence of atomic isotopic ratios as small as 10xe2x88x9215, for example between carbon-12 and carbon-14. The detection of very small quantities of isotopes from samples of less than 1 mg has revolutionized the process of carbon dating. The ability to uniquely detect the presence of atomic isotopes finds many uses, for example, carbon dating, or using atomic isotopes as chemical labels. The use of long-lived radioactive compounds as labels forms an important subset of the possible uses to which accelerator mass spectrometry (AMS) can be employed. Radioactive isotopes with long half-lives are difficult to measure by detection of radioactive decay if the sample size is small and the half-life of the radioactive isotope is large. For radioactive carbon-14, with a half-life of 5,730 years, a sample size of one gram is generally considered necessary for radioactive carbon dating. A one-gram sample of modern carbon contains approximately 10xe2x88x9212 grams 14C or approximately 5xc3x971010 atoms of 14C and produces only 14 disintegrations per minute. Using an accelerator mass spectrometer (AMS) as much as 10 percent of the atoms of 14C present in a sample can be directly detected. The result is that the concentration of carbon-14 can be measured with a precision of better than one percent in a modern sample, using a sample size of less than one mg in only a few minutes.
Mass spectrometry uses the principal that a charged particle is deflected more or less by a magnetic or static electric field depending on the velocity and mass of the particle. By the proper combination of magnetic and/or electrostatic analyzers it is possible to separate particles by mass and velocity and thus to detect the mass and energy of individual particles. The detection of a particular atomic isotope, however, requires for unique detection that all molecular isobars be eliminated. For example, in the case of carbon-14 molecular isobars of 13CH and 12CH2 are perhaps one million times more prevalent than the carbon-14 to be measured. To detect carbon-14, negatively charged particles of mass 14 are accelerated in the tandem accelerator through a potential of about one-half million volts to several million volts. The negatively charged particles of mass 14 are passed through a stripping column of rarefied gas in the high voltage positively charged electrode. The stripping column causes the particles to lose electrons and in the process breaks up any molecular isobars into their constituent parts. The positively charged ions are accelerated away from the positively charged high voltage electrode to ground and the particles of mass 14 are separated and counted.
Although very successful accelerator mass spectrometers (AMS) are relatively expensive and of large size, and have certain operation requirements such as the handling of sulfur hexafluoride insulating gas which contribute to the expensive operation. A smaller and simpler design for an accelerator mass spectrometer (AMS) is needed to facilitate the continued growth of AMS applications.
The accelerator mass spectrometer of this invention utilizes a single stage air insulated accelerator (SSAMS). A negative carbon ion source is placed inside a negatively-charged high voltage terminal. The ion beam emerges from the ion source and is accelerated to moderate energy, approximately 35,000 electron volts, and is filtered by a momentum analyzer, i.e., an analyzing bending magnet, to remove unwanted ions. Reference ions such as carbon-12 are deflected and measured in an off-axis Faraday cup. Ions of mass 14 are accelerated to ground potential and passed through a gas stripper where the ions undergo charge exchange and molecular destruction. The desired isotope, carbon-14 along with fragments of the interfering molecular ions emerge from a stripper into a momentum analyzer (analyzing bending magnet) which removes all but the desired isotope ions from the beam. The ions in emerging from the analyzing magnet are further filtered by passing through an electrostatic spherical analyzer to remove ions which have undergone charge exchange while passing through the analyzing magnet. The ions remaining after the spherical analyzer are transmitted to a detector and counted.
It is an object of the present invention to provide an accelerator mass spectrometer of lower-cost, simpler operation and smaller size.
It is a further object of the present invention to provide an accelerator mass spectrometer for detecting carbon-12 to carbon-14 ratios.
It is another object of the present invention to provide an accelerator mass spectrometer utilizing an air insulated high voltage electrode.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.